The "Oh, hello," video is a type of video - often an infomercial, a documentary, an introduction to masterpiece theater, etc. - in which the narrator is in his natural habitat doing whatever and then looks surprised when the camera starts rolling, as if the viewer is in the room and is unexpectedly visiting. The narrator then launches into some sort of classy and informative monologue.
In addition to the surprised "Oh, hello" beginning, here are some other features that good "Oh, hello" videos often have...
1) The subject is in a dimly-lit study, surrounded by books.
2) The subject is middle-to-late aged and male.
3) The subject is wearing a blazer or leisure jacket.
4) The subject is smoking a pipe, cigar, cigarette, or, if need be, a joint.
4) The subject is drinking brandy or scotch.
5) The subject has a pleasant, comforting, classy and witty affect.
To give you a concrete example of a very good "Oh, hello" video, here is an extremely well-done example with Billy Dee Williams, who, of course, played Lando in Empire Strikes Back and Return of the Jedi.
*To note, Billy Dee Williams does not actually say "Oh, hello," - he says "Oh,hi! I'm Billy Dee Williams." This is completely acceptable. As long as the narrator seems surprised and takes a serious attitude towards his speaking material, it's totally fine.